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Sound, with its unique properties, behaves differently in various environments. Our three-part series of articles takes a comprehensive approach to delve deeper into this complex and fascinating phenomenon. Each part will focus on one or two of the four key approximations to the problem’s interpretation, providing a thorough understanding of the intricate and multifaceted nature of sound wave propagation.  

Sound propagation through a valley below a dam depends on the terrain’s geomorphology. This analysis examines the extent to which natural valley characteristics can affect the range and effectiveness of an acoustic warning signal. It also demonstrates that the designer can rarely count on the contribution of the valley’s morphology to the quality of this signal. 

Exclude all influences participating in the propagation of a warning signal except for the geomorphology of the terrain. In this null approximation, the acoustic properties of the valley slopes will not be considered either, and the assumptions are that 

• the valley is rocky and 

• the valley is straight with perpendicular walls/slopes. 

When evaluating the impact of the valley’s morphology, a certain benchmark or reference condition must be set. Let us consider a hypothetical isotropic sound source hanging freely over a hill with monotonous slopes similar to an idealised valley. This reference condition will help us gauge the valley’s influence on sound wave propagation. 

The first situation is illustrated in Figure 1. 

Now imagine the valley’s upper end below the dam, as illustrated in Figure 2. 

If there were only a sloping plane below a dam and no valley, there would be no difference in the sound wave propagation conditions, as shown in the figures above.  

The differences in the sound propagation conditions allow for a comparison of the horizontal views displayed in Figures 3, 4 and 5. 

It is clear from the figures above that a theoretical ‘canyon effect’ is at play here. This effect essentially means that the valley acts as a sound tunnel.  This is particularly true for areas further away from the dam wall where the propagating sound wavefront is expected to be perpendicular to the valley’s slope (as explained later). In simpler terms, the sound wave (represented by an imaginary vector) does not bounce off the bottom of the valley. Instead of being directed into an open space, the sound wave is directed into the valley. This is a rare case of a long, straight valley with perpendicular walls.  

Sooner or later, the sound wave vectors will hit the flooded or unflooded, vegetated valley bottom (the negative sound scattering effect must be considered for the vegetated valley) and bounce irreversibly upwards, as demonstrated in Figure 6. 

Let us conclude the null approximation as follows:  

As the residues of sound wave energy are distant enough from the dam wall, they do not affect the direction of sound wave propagation. Hence, it remains parallel to the walls of the straight valley bottom, which can account for this theoretical canyon effect. However, it concerns only a fraction of the total amount of the radiated energy of a sound wave.  

However, this is not a purely hypothetical case. This first approximation to the interpretation of the problem can correspond to actual natural conditions, such as dams in the Colorado River valley above Las Vegas. This region is known for its unique geomorphology, characterised by long, straight valleys with perpendicular walls and sound propagation characteristics, as shown in Figure 7. 

Let us conclude the first part of this article with a layperson’s summary. This summary will reinforce the central insights of our analysis, particularly the significant influence of the terrain’s shape and character on the propagation of a sound wave below a dam. This is a crucial understanding for those involved in designing or implementing warning systems in such environments.  

The following article will tackle two further approximations to the interpretation of the same problem in more detail and will refer back to the canyon effect. 

The article was written by

Stanislav Gašpar

Stanislav worked in electronics design for a long time before transitioning to acoustics, bringing a nonconformist approach to addressing related topics. Recently, in the context of acoustics, he finds it stimulating to engage with AI, aiming to make it contradict itself and impose his own interpretation of the presented problem. Through years of experience in the technocratic industry, he has come to embrace two guiding principles: reality is orders of magnitude more complex than we interpret it, and the real fun begins when “something doesn’t work.” Additionally, he enjoys expressing his thoughts on poetry and music.